Bis(purine) complexes of trans-a₂Pt^II: Preparation and x-ray structures of bis(9-methyladenine) and mixed 9-methyladenine, 9-methylguanine complexes and chemistry relevant to metal-modified nucleobase triples and quartets

Mononuclear bis(purine) complexes of trans-a₂Pt^II, trans-[(NH₃)₂Pt(9-MeA-N7) (9-MeGH-N7)](NO₃)₂·H₂O (1b) and trans-[(NH₃)₂Pt(9-MeA-N7)₂](ClO ₄)₂·H₂O (3c), have been prepared and their structures determined by X-ray crystallography. 1b: Space group P1, a = 7.245 (5) Å, b = 7.715(6) Å, c = 10.907(8) Å, α = 82.36(6)°, β = 86.62(6)°, γ = 70.15(6)°, V = 568.3(7) ų, Z = 1. 3c: Space group P2₁/c, a = 8.312(2) Å, b = 15.386(3) Å, c = 12.365(2) Å, β = 94.83(3)°, V = 1575.72(55) Å, Z = 2. The cation of 3c is centrosymmetric. In the cation of 1b, the two purines adopt a head-head orientation with an intramolecular H bond of 2.94(3) Å between the exocyclic amino group of 9-MeA and the exocyclic carbonyl group of 9-MeGH. Di- and trinuclear derivatives of 1b and 3c have been synthesized and/or studied in solution. They include compounds of types Cl•N1-A-N7•GH (2a), T•N1-A-N7•GH (2b), GH•N1-A-N7•T (2c), and GH•N7-A-N1•N1-A-N7•GH (2d) as well as X•N1-A-N7•N7-A-N1•X (X = Cl (4), GH (5a), T (5b)) with • representing trans-a₂Pt^II entities (a = NH₃ or CH₃NH₂). The fact that Pt-(A-N1) and Pt-(A-N7) vectors are at right angles and the nucleobases essentially coplanar in many cases leads to intramolecular H bonding involving NH₂(6) of 9-MeA and exocyclic groups of the other nucleobases. The chemical shifts of the NH₃ protons of 9-MeA in DMSO-d₆ or DMF-d₇ permit a differentiation between the various possibilities (no H bonding, single H bond, 2 H bonds, 1 Pt or 2 Pt coordinated to 9-MeA). As far as intermolecular H bonding is concerned, the neutral 9-MeGH ligand in 1b forms a Watson-Crick pair with 1-MeC but a 9-MeGH = 9-MeG pair at pH 8. The potential usefulness of the complexes prepared with regard to the formation of two-dimensional sheet structures and molecular squares built up of purine nucleobase and trans-a₂Pt^II entities is briefly discussed, as are aspects of the stabilization of triplex nucleic acid structures by metal ions.